1. Field of the Invention
The present invention relates to battery equalization techniques and, more particularly, to magnetically-coupled battery equalization apparatus suitable for autonomous connection to batteries.
2. Related Art
To generate higher voltage than available from a single battery, plural batteries are typically connected in series such that a relatively large total voltage is available to drive a load. As it is desirable to utilize rechargeable batteries, battery charger circuits have been developed which charge all of the batteries in a series string at one time.
Care must be taken to fully charge each battery in the series string without one battery being at a higher state of charge than another battery. If a difference exists between a relatively low charge on one battery with respect to the other batteries in the series, the total effective capacity of the series string of batteries is reduced to the capacity of the battery having the low state of charge.
Battery equalization circuits have been developed to ensure that all batteries in a series string attain substantially the same state of charge. U.S. Pat. No. 5,479,083 to Brainard illustrates a conventional battery equalization circuit which includes a pair of series-coupled transistors connected across a pair of series-coupled batteries. An inductor L is connected between the pair of transistors and the batteries. An oscillator produces gate drive signals to the transistors such that they are alternately biased on and off for substantially equal durations. The inductor operates as a non-dissipative shunt that is alternately switched in parallel with each battery such that excessive charge on one battery is transferred to the other battery. Unfortunately, component tolerances within the Brainard equalization circuit will effect the degree of equalization achieved between the batteries, particularly tolerances which effect the duty cycle of the oscillator and the resultant duty cycle presented by the transistors to the batteries. Therefore, in order to obtain satisfactory equalization, measurements of the charge on each battery must be obtained and fed-back to the oscillator to change the duty cycle as necessary (see FIG. 3 of the Brainard patent).
U.S. Pat. No. 5,710,504 to Pascual discloses a battery equalization circuit which does not require a feedback mechanism from each battery to achieve adequate equalization. However, the circuit of the Pascual patent requires that all switching devices within the circuit be synchronized, no matter how many batteries are in the series combination. When the number of series-coupled batteries is relatively high and results in a high terminal voltage from the uppermost battery to the lowermost battery, the topology of the Pascual circuit may result in undesirable fault conditions.
Turning to FIG. 1 of the Pascual patent, a plurality of series-coupled batteries are shown; all switches 16 are synchronized via control lines 18 and control unit 12. Assuming that the total voltage from the uppermost battery to the lowermost battery is substantially large (e.g., 600 volts), a practical circuit must be designed to withstand a fault from the uppermost battery terminal to the lowermost battery terminal through the wiring of the equalization circuit. Often, the series coupled batteries may deliver many amps (approaching 1000 amps or more) making it difficult to design for surviving a fault and not damaging any of the batteries.
U.S. Pat. No. 5,821,729 to Schmidt discloses a method and apparatus for exchanging charge between a plurality of batteries, where transformer windings are connected in parallel with each battery at predetermined time intervals. Each battery is simultaneously connected to a respective one of the windings in the same winding sense. Unfortunately, the Schmidt apparatus requires precise timing of switching elements which connect the windings to the respective batteries. Indeed, if the timing of the switching elements is not tightly controlled, the common magnetic core of the transformer windings will saturate.
Accordingly, there is a need in the art for a new battery equalization circuit which is capable of autonomous operation (i.e., not requiring synchronization with other equalization circuits servicing the series-coupled batteries) and does not require closed loop compensation or overly complex control circuitry to achieve satisfactory equalization.